Tissue microarrays are a method of harvesting small discs of tissue from a range of standard histologic sections and arranging them on a recipient paraffin block such that hundreds or thousands of discs can be analyzed simultaneously. This technique allows maximization of tissue resources by analysis of small core biopsies of blocks, rather than complete sections. A carefully planned array of tissues can be constructed with cases from pathology tissue block archives, such that a 20-year survival analysis can be performed on a cohort of 600 or more patients by use of only a few microliters of antibody.
Tissue microarray technology has numerous advantages in addition to tissue amplification. For example, each specimen is treated in an identical manner. Like conventional formalin-fixed paraffin embedded material, tissue microarrays are amenable to a wide variety of techniques, including histochemical stains, immunologic stains with either chromogenic or fluorescent visualization, in situ hybridization (including messenger RNA in situ hybridization and fluorescence in situ hybridization) and even microdissection techniques. For each of these protocols conventional sections can have substantial slide-to-slide variability associated with processing 300 slides (e.g. 20 batches of 15 slides). By contrast, the tissue microarrays allow an entire cohort to be analyzed on a single slide. Thus, reagent concentrations are identical for each case, as are incubation times and temperatures and wash conditions. Antigen retrieval can be another significant variable in conventional sections, which is mitigated by the identical treatment of specimens in a microarray. As a further advantage, only a few microliters of reagent may be required to analyze an entire cohort in a microarray. This advantage raises the possibility of using tissue microarrays in certain screening procedures, such as hybridoma screening, where the protocol is not amenable to the use of conventional sections.
Currently, the primary method used to evaluate microarrays involves manual review of hundreds of tissue microarray (“TMA”) cores under a microscope, while subjectively evaluating and scoring the signal at each location. An alternate, but less utilized approach is to sequentially digitize specimens for subsequent assessment. Both procedures involve manually and systematically reviewing the TMA sample under the microscope, which is a slow, tedious process, and which is especially error-prone because it is easy to loose track of a current array while navigating among the regularly arranged specimens. This is especially true at higher (e.g. 20×) magnifications.
Tissue microarrays also present some special problems such as heterogeneity of tissue sections, sub-cellular localization of staining, and background signal. Depending on the type of tumor or tissue section analyzed, the area of interest may represent nearly the entire disc or only a small percentage thereof. For example, a pancreatic carcinoma or lobular carcinoma of the breast with substantial desmoplastic response may show stromal tissue representing a large percentage of the total area of the disc. If the goal of the assay is to determine epithelial cell expression of a given marker, a protocol must be used that evaluates only that region of the disc. The protocol must not only be able to select the region of interest but also normalize it so that the expression level read from any given disc can be compared with that of other discs. Sub-cellular localization presents a host of additional challenges when comparing nuclear or membranous stainings which are quite different from those in total cytoplasmic staining.
There remains a need for a systematic approach to collecting, analyzing, and storing data from tissue microarrays.